The steps in processing semiconductor wafers into individual integrated circuits (ICs) are well known in the art. In one widely used manufacturing method a semiconductor wafer (typically six or eight inches in diameter) is chemically and photographically processed through a number of steps to produce a multitude of very closely spaced and precisely detailed ICs on the wafer. In such processing, a wafer may be exposed within a reactor to a highly active plasma of special gas or gases in order to etch, by means of reactive ions of the gases, very fine details (lines, zones, etc.) into a top surface of a wafer being processed. The wafer is subsequently cut into individual ICs. This general technology is well known in the art and need not be described further.
A typical IC processing apparatus comprises a reactor in which there is a chamber through which reactive gas or gases are flowed, a support such as a chuck or pedestal for holding one or more wafers in proper position in the chamber to expose a top surface of the wafer to the reactive gas.
By way of example, during etching in a typical plasma etching operation, the reactive ions of the plasma gas chemically react with portions of material on a face of the semiconductor wafer. This process is exothermic and causes some degree of heating of the wafer. The chemical reaction between gas (ions and radicals) and wafer material, on the other hand, is accelerated to some degree by the temperature rise of the wafer. Wafer temperature and the rate of chemical reaction are related to an extent that harmful over-etching (or under-etching) of material over a face of the wafer can easily result if the temperature of the wafer varies too much from a desired value. It is highly desirable therefore that wafer temperature be accurately controlled with respect to a given process e.g., reactive ion etching (RIE)! since otherwise the IC's being fabricated on the wafer will have electronic characteristics which deviate from their design center more than is desirable.
The problem of temperature rise of a wafer during IC processing is well known. One effective way to control wafer temperature to a desired value is to flow coolant gas (such as helium) at a suitable pressure and flow rate within and through a thin space between the bottom of the wafer and the top of the chuck or pedestal which holds the wafer. However, there are certain difficulties in measuring the temperature of the wafer during processing. To put a temperature probe, such as a thermocouple, in actual contact with the wafer can cause distortion in the temperature distribution over the wafer itself and this in turn affects measurement accuracy as well as the processing of the wafer. On the other hand, remote sensing of the temperature of the wafer by infrared (IR) scanner is complicated by the fact that the wafer, being a thin piece of essentially pure silicon, is transparent to certain wavelengths of infrared light (much as an ordinary glass window is transparent to visible light). An IR sensor therefore "sees" not only the wafer temperature but that of the chuck on which it is mounted and also possibly background temperature of the walls of the reaction chamber within which the wafer is being processed. This makes it necessary to carefully calibrate the IR sensor with respect to the heat emissivity of a wafer being processed as well as that of the chuck and possibly also the inside walls of the reaction chamber.
From the above discussion, it is desirable to provide easy and accurate measurement and control of the temperature of a wafer during processing.